Lithium secondary battery

ABSTRACT

The present invention provides a secondary battery which is less expensive, whose safety is extremely high, which is of large capacity as well as good cyclic characteristic, and which uses an aqueous solution for the electrolytic solution. A lithium secondary battery according to the present invention is constituted by including a positive electrode, formed by binding a positive-electrode raw-material mixture including a positive electrode active material, a negative electrode, formed by binding a negative-electrode raw-material mixture including a negative electrode active material, and an electrolytic solution, comprising an aqueous solution in which a lithium salt is dissolved, said positive electrode active material including at least one of a layered structure lithium-manganese composite oxide whose basic composition is LiMnO 2  and an olivine structure lithium-iron composite phosphorus oxide whose basic composition is LiFePO 4 . The present lithium secondary battery can be used in the fields of communication appliances and information-related appliances, and as an electric source for electric automobiles and the like.

TECHNICAL FIELD

The present invention relates to a lithium secondary battery in whichthe dope-undope phenomenon of lithium is utilized, and particularlyrelates to an aqueous lithium secondary battery which includes anelectrolytic solution comprising an aqueous solution.

BACKGROUND ART

Since lithium secondary batteries in which the dope-undope phenomenon oflithium is utilized exhibit high energy densities, as cellular phones,personal computers and the like have been downsized, they have beenwidely spread in the fields of communication appliances andinformation-related appliances. Moreover, in the field of automobiles aswell, it has been urged to develop electric automobiles because of theenvironmental problems as well as the resource problems, as an electricsource for electric automobiles as well, lithium secondary batterieshave been investigated.

Lithium secondary batteries, which have been currently put to practicaluse, are generally constituted by a positive electrode which uses alithium-transition metal composite oxide as a positive electrode activematerial, a negative electrode which uses a carbon material and the likeas a negative electrode active material, and a nonaqueous electrolyticsolution in which a lithium salt is dissolved in an organic solvent, andthose which exhibit high voltages of 4 V-class make a main stream.

However, since the above-described lithium secondary batteries usenonaqueous organic solvents whose burning points are low as theelectrolytic solutions, the safety matters. For example, in case ofarriving at over charged states, and in case of being exposed to hightemperature environments, for the purpose of securing safety, it isgeneral to equip them with devices such as PTC elements and safetyvalves. However, since combustible solvents are used, in order to fullysecure safety, considerable difficulties follow them about. Inparticular, secondary batteries as an electric source for poweringautomobiles and the like are big, and the amounts of used organicsolvents are large, in addition thereto, it is expected to use themunder severe conditions such as the service temperatures, much highersafety is required.

Moreover, when moisture is present even in a small amount in batteries,there arise various problems such as the generation of gases by means ofthe electrolysis reaction of water, the consumption of lithium by meansof the reaction between water and lithium, the corrosion of batteryconstituent materials. Accordingly, in the production of lithiumsecondary batteries, a thoroughly dry environment is required, specialequipment for completely removing moisture and a large amount of laborare needed, and this is one of the causes for pushing up the costs ofbatteries.

Meanwhile, in aqueous lithium secondary batteries which use aqueoussolutions as the electrolytic solution, the aforementioned problems donot arise basically. Moreover, in general, since aqueous solutions areof better conductivity compared to nonaqueous solutions, the reactionresistance of batteries also decreases, and the power characteristic andrate characteristic of batteries are improved. However, since it isnecessary to charge and discharge in a potential range where theelectrolysis reaction of water does not occur, the aqueous lithiumsecondary batteries suffer from a drawback in that it is difficult,compared to nonaqueous lithium secondary batteries, to secure a largedischarge capacity.

From this, in aqueous lithium secondary batteries, it is desired to usean active material which is not only stable in aqueous solutions butalso can reversibly dope-undope lithium ions in a large amount in apotential range where oxygen and hydrogen are not generated by means ofthe electrolysis of water, namely which exhibits a large capacity.

As for aqueous lithium secondary batteries which have beenconventionally investigated, there exist, for example, as disclosed inPublished Japanese Translation of PCT International Publication forPatent Application No. 9-508,490, a battery which uses LiMn₂O₄ and thelike as the positive electrode active material and LiMn₂O₄, VO₂ and soforth as the negative electrode active material, and, moreover, asdisclosed in Japanese Unexamined Patent Publication (KOKAI) No.12-77,073, a battery which uses LiCoO₂, Li(Ni, Co)O₂, LiMn₂O₄ and thelike as the positive electrode active material and LiV₃O₈ and so forthas the negative electrode active material.

When the present inventors carried out various tests while payingattention to the active materials, it was found out that it is difficultfor LiCoO₂, Li(Ni, CO)O₂, LiMn₂O₄ and the like, which are positiveelectrode active materials having been investigated conventionally, andfor LiV₃O₈, VO₂ and so forth, which are negative electrode materials, totake out a sufficient capacity in a potential range where theelectrolysis reaction of water does not occur, and that they furtherhave a problem as well in terms of the stability in aqueous solutions.Therefore, in case of actually constituting aqueous lithium secondarybatteries by using them, the capacities and cycle characteristics of thelithium secondary batteries do not become practically satisfactory ones.

DISCLOSURE OF THE INVENTION

The present invention has been done in view of the aforementionedproblems, and it is an assignment, in an aqueous lithium secondarybattery which uses an aqueous solution as the electrolytic solution, todiscover a positive electrode active material which can take out asufficient capacity, and moreover to provide an aqueous lithiumsecondary battery whose capacity is large by constituting a battery bycombining an appropriate negative electrode active material therewith.

As a result of further studies on aqueous lithium secondary batteries inorder to solve this assignment and a great number of experimentsthereon, the present inventors arrived at discovering an active materialwhich has a large capacity within the cell voltage ranges of aqueouslithium secondary batteries.

Namely, a lithium secondary battery according to the present inventionis constituted by including a positive electrode, formed by binding apositive-electrode raw-material mixture including a positive electrodeactive material, a negative electrode, formed by binding anegative-electrode raw-material mixture including a negative electrodeactive material, and an electrolytic solution, comprising an aqueoussolution in which a lithium salt is dissolved, said positive electrodeactive material including at least one of a layered structurelithium-manganese composite oxide whose basic composition is LiMnO₂ andan olivine structure lithium-iron composite phosphorus oxide whose basiccomposition is LiFePO₄.

The present inventors examined the charge-discharge behaviors of mainactive materials, and discovered that a lithium-manganese compositeoxide, whose basic composition is LiMnO₄ and which has a layeredstructure, and an olivine structure lithium-iron composite phosphorusoxide whose basic composition is LiFePO₄ can, in a potential range wherethe oxygen generation by means of the electrolysis of water does notarise, dope-undope lithium ions in a large amount reversibly, and thatthey are suitable as the positive electrode active material in aqueouslithium secondary batteries. Namely, the lithium-manganese compositeoxide and the lithium-iron composite phosphorus oxide, being thepositive electrode active material of the lithium secondary batteryaccording to the present invention, can, in a potential range where theoxygen generation by means of the electrolysis of water does not arise,take out a capacity sufficiently.

Here, in FIG. 1, there are illustrated the relationships between thecapacities of a variety of representative lithium-transition metalcomposite oxides, which can be used as the positive electrode activematerial, and the potentials (vs. Li/Li⁺). As it is apparent from FIG.1, LiCoO₂ and LiMn₂O₄ cannot, in a potential range where the oxygengeneration by means of the electrolysis of water does not arise, takeout capacities so much, and Li(Ni, Co)O₂ as well remains at about halfof the inherent capacity. Note that, in actuality, since a trace amountof Li turns into LiOH in water to dissolve therein and the electrolyticsolution tends to be alkaline so that the oxygen generation potentiallowers, it becomes more severe conditionally. Meanwhile, since LiMnO₂and LiFePO₄ can, in a potential range where the oxygen generation bymeans of the electrolysis of water does not arise, dope-undope lithiumions in a large amount reversibly, they can take out capacitiessufficiently. Therefore, it is possible to confirm that the lithiumsecondary battery according to the present invention which uses LiMnO₂or LiFePO₄ as the positive electrode active material makes a secondarybattery of large capacity.

Therefore, the lithium secondary battery according to the presentinvention makes an aqueous secondary battery which is less expensive,whose safety is extremely high, and which is of high power as well aslarge capacity. Moreover, although having been revealed in thesubsequent experiments, it makes an aqueous secondary battery whichlowers the capacity less even after repeating charge-discharge, andwhich is good in terms of cycle characteristic, in particular, cyclecharacteristic at high temperatures.

Moreover, the lithium secondary battery according to the present candesirably be embodied such that a lithium-vanadium composite oxide ortransition metal chalcogenide, which is a substance having a lithiumdope-undope potential lower than those of the aforementionedlithium-manganese composite oxide and lithium-iron composite phosphorusoxide, is included in the negative electrode active material. Forexample, among the lithium-vanadium composite oxides, it is desirable touse a lithium-vanadium composite oxide which has, in an X-raydiffraction pattern by means of CuKα ray, the highest intensity peak at2θ=13.9°±1° (θ being diffraction angle), and in which the intensity ofthe peak is 5 times or more compared to the intensities of all of theother peaks.

The aforementioned lithium-vanadium composite oxide being suitable forthe negative electrode active material has not been made clear atpresent on what space group its crystalline structure has, because theintensities of most of the peaks, which are recognizable from the X-raydiffraction pattern, are small. Therefore, the aforementionedlithium-vanadium composite oxide can only be defined such that it hasthe distinctive X-ray diffraction pattern as described above. As anexample of the X-ray diffraction pattern, an X-ray diffraction chart ofa lithium-vanadium composite oxide, used in a lithium secondary batteryof a latter-described experimental example, by means of CuKα ray isillustrated in FIG. 2.

As illustrated in FIG. 2, in the X-ray diffraction chart, there is thehighest intensity peak at 2θ=13.9°±1° (θ being diffraction angle), andthe intensities of the other peaks, excepting the peak, are extremelylow. The intensity of the highest intensity peak is an intensity of 5times or more as much as the intensities of all of the other peaks. Fromthis X-ray diffraction chart, it is possible to assume and judge thatthe present lithium-vanadium composite oxide has a crystalline structurewhich has a space group being strongly oriented in one direction.

On the other hand, in a lithium-vanadium composite oxide, beingexpressed by a composition formula LiV₃O₈, which has been investigatedconventionally, an X-ray diffraction chart as illustrated in FIG. 3 isobtained. By comparing the X-ray diffraction chart of FIG. 2 to theX-ray diffraction chart of FIG. 3, the peculiarity of the crystallinestructure of the present lithium-vanadium composite oxide is apparent.

It is possible to assume and judge that the fact that the presentlithium-vanadium composite oxide exhibits a good characteristic as thenegative electrode active material of aqueous lithium secondarybatteries results from having the above-described specific crystallinestructure, but, at present, the accurate reasons have not been madeclear. However, according to later-described experiments, in theoperating cell voltage ranges of aqueous lithium batteries, largecapacities are surely obtained, this is believed that, in the presentlithium-vanadium composite oxide, the constituent atoms are disposed sothat they are oriented in one direction, and, resulting therefrom, thedope-undope of lithium is carried out with ease.

Further, since the charge-discharge curve shows an extremely flat curve,it is believed that the present lithium-vanadium composite oxide is onewhich does not undergo phase transition within the operating cellvoltage ranges of aqueous lithium secondary batteries, namely one whichhas a crystalline structure free from accompanying phase transition.From this as well, it is possible to recognize that the presentlithium-vanadium composite oxide can obtain large capacities within theoperating cell voltage ranges and makes a suitable negative electrodeactive material. Furthermore, its charge-discharge cycle characteristicis good, it is stable with respect to aqueous electrolytic solutions,moreover, its crystalline structure is hardly broken down by repetitivecharge-discharge, and it makes a negative electrode active materialwhich can maintain the large capacities.

As for the lithium-vanadium composite oxide, in addition to theaforementioned lithium-vanadium composite oxide, it is desirable to usea spinel structure lithium-vanadium composite oxide whose basiccomposition is LiV₂O₄. As it will be described in detail later, when anelectrode was manufactured in which the present lithium-vanadiumcomposite oxide made an active material and a single electrodeevaluation test by means of a cyclic voltamogram was carried out, it wasfound out that, from the obtained current-potential curve (CV curve),the present lithium-vanadium composite oxide shows a so-called 2-phasecoexisting lithium dope-undope behavior which has one and onlyoxidation-reduction potential.

Namely, different from LiNiO₂ and LiCoO₂ in which the lattice constantsand the like vary continuously with the dope-undope of lithium to varypotentials in charge-discharge, the present lithium-vanadium compositeoxide shows a substantially constant potential in charge-discharge.Then, the potential of charge-discharge is at around 2.4 V (vs. Li/Li⁺),and this potential is within a potential range where the hydrogengeneration by means of the electrolysis of water does not arise.Moreover, since the rising of the peak in the obtained CV curve is steepand the polarization is less, it was found out that the reactionresistance in the oxidation-reduction reaction is small. From this, thepresent lithium-vanadium composite oxide can, in a potential range wherethe hydrogen generation by means of the electrolysis of water does notarise, dope-undope a large amount of lithium ions, and, in particular,it is possible to say that it is suitable as the negative electrodeactive material in aqueous lithium secondary batteries.

Therefore, the lithium secondary battery according to the presentinvention which is embodied to use the aforementioned lithium-vanadiumcomposite oxide as the negative electrode active material makes alithium secondary battery which exhibits a large capacity and whosecyclic characteristic is good.

As having described so far, in the present description, the presentinvention has been described as lithium secondary batteries, however,the distinctive features lie in the respective active-material rawmaterials which constitute the positive electrode and negativeelectrode. Therefore, it is possible to grasp the present invention forthe every aforementioned respective active-material raw materials, forexample, a lithium-manganese composite material for the positiveelectrode active material of lithium secondary batteries, a lithium-ironcomposite phosphorus oxide for the positive electrode active material oflithium secondary batteries, a lithium-vanadium composite oxide for thenegative electrode active material of lithium secondary batteries, andthe like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the relationships between the capacities of a varietyof representative lithium-transition metal composite oxides, which canbe used as the positive electrode active material, and the potentials(vs. Li/Li⁺).

FIG. 2 illustrates an X-ray diffraction chart by means of CuKα ray on alithium-vanadium composite oxide which was used in a secondary batteryof an experimental example and is expressed by a composition formulaLi_(1.5)V₃O_(7.8-8).

FIG. 3 illustrates an X-ray diffraction chart by means of CuKα ray on alithium-vanadium composite oxide which has been investigated as anegative electrode active material conventionally and is expressed by acomposition formula LiV₃O₈.

FIG. 4 illustrates a CV curve, which was obtained in a single electrodeevaluation test by means of a cyclic voltamogram, on an olivinestructure lithium-iron composite phosphorus oxide.

FIG. 5 illustrates a charge-discharge curve, which was obtained by asingle electrode evaluation test, on a lithium-vanadium compositephosphorus oxide which is expressed by a composition formulaLi_(1.5)V₃O_(7.8-8).

FIG. 6 illustrates a CV curve, which was obtained in a single electrodeevaluation test by means of a cyclic voltamogram, on a spinel structurelithium-vanadium composite oxide which is expressed by a compositionformula LiV₂O₄.

FIG. 7 illustrates CV curves, which were obtained in a single electrodeevaluation test by means of a cyclic voltamogram, on a spinel structurelithium-vanadium composite oxide which is expressed by a compositionformula LiV₂O₄ and on an olivine structure lithium-iron compositephosphorus oxide.

FIG. 8 illustrates charge curves on lithium secondary batteries of #15and #16.

FIG. 9 illustrates discharge capacities on a lithium secondary batteryof #11 in respective cycles at 20° C.

FIG. 10 illustrates discharge capacities on lithium secondary batteriesof #11 and #14 in respective cycles at 60° C.

FIG. 11 illustrates discharge capacities per unit weight of positiveelectrode active materials on secondary batteries of #21 and #22 inrespective cycles at 20° C.

FIG. 12 illustrates discharge capacities per unit weight of positiveelectrode active materials on secondary batteries of #31 through #33 inrespective cycles at 60° C.

FIG. 13 illustrates discharge capacities per unit weight of positiveelectrode active materials on secondary batteries of #41 through #45 inrespective cycles at 60° C.

BEST MODE FOR ENFORCING INVENTION

Hereinafter, embodiment modes of the lithium secondary battery accordingto the present invention will be described for the every respectiveconstituent elements.

<Positive Electrode Active Material>

The lithium secondary battery according to the present inventionincludes, as the positive electrode active material, at least one of alayered structure lithium-manganese composite oxide whose basiccomposition is LiMnO₂ and an olivine structure lithium-iron compositephosphorus oxide whose basic composition is LiFePO₄.

Here, the basic composition means the respective representativecompositions of the lithium-manganese composite oxide and lithium-ironcomposite phosphorus oxide, in addition to those expressed by theaforementioned composition formulas, those compositions are included aswell in which the other one or two or more atoms of Co, Ni, Al, Mg andthe like, for example, substitute and so forth for part of the lithiumsites, manganese sites and iron sites. Moreover, it is not necessarilylimited to those with the stoichiometric compositions, and those withnon-stoichiometric compositions and so on, which arise inevitably in theproduction, are included as well in which, for instance, cations of theatoms, such as lithium, manganese and iron, are omitted or the oxygenatoms are omitted.

Further, as for the lithium-manganese composite oxide having a layeredstructure, there are a lithium-manganese composite oxide (space groupR3m) having a hexagonal layered structure, a so-called layered rocksaltstructure, a lithium-manganese composite oxide (space group C2/m) havingan orthorhombic layered structure and a lithium-manganese compositeoxide (space group Pmnm) having a monoclinic zigzag layered structure,one member among them can be used independently, and, furthermore, twoor more members can be mixed to use.

Among them, it is desirable to use the lithium-manganese composite oxidehaving a hexagonal layered rocksalt structure. The lithium-manganesecomposite oxide having a hexagonal layered rocksalt structure is suchthat, even when charge-discharge is repeated, the transformation to thespinel structure from which capacity is taken out less does not arise,and accordingly the lithium secondary battery according to the presentinvention which uses this as the positive electrode active materialmakes an aqueous secondary battery of much larger capacity.

Moreover, the lithium-iron composite phosphorus oxide is such that itscrystalline structure is made into an orthorhombic olivine structure,and its space group is expressed by Pmnb. Namely, the olivine structureis a structure which is based on the hexagonal close-packed structure ofoxygen, in which phosphorus is positioned at the tetrahedron sites, andin which both of lithium and iron are positioned at the octahedronsites.

The present lithium-iron composite phosphorus oxide is such that theparticle diameters of its particles are not limited in particular,however, from the viewpoint of smoothly carrying out the reaction ofdope-undope lithium ions to obtain a sufficient active materialdischarge capacity in case of doing charge-discharge at a practicalcharge-discharge density, it is desirable to arrange the averageparticle diameter of its particles to be 1 μm or less. In particular,taking the readiness of manufacturing battery and exhibiting good ratecharacteristic into consideration, it is desirable to arrange theaverage particle diameter to be from 0.2 μm or more to 0.8 μm or less.

Note that the lithium-iron composite phosphorus oxide is formed fromparticles which exist substantially independently. Therefore, theaverage particle diameter is the average value of the particle diametersof the particles which exist substantially independently, the respectiveparticle diameters can be measured, for example, by using a scanningelectron microscope (SEM) photograph on the lithium-iron compositephosphorus oxide. Namely, an SEM photograph on the lithium-ironcomposite phosphorus oxide is taken, the maximum diameters and minimumdiameters of the lithium-iron composite phosphorus oxide particles inthe photograph are measured, and the average value of those two valuescan be employed as the particle diameter of one particle of them.

Note that the aforementioned lithium-manganese composite oxide having ahexagonal layered rocksalt structure and lithium-iron compositephosphorus oxide having an olivine structure are such that theirproduction methods are not limited in particular, however, in accordancewith later-described production methods discovered by the presentinventors, these can be produced with ease.

<Negative Electrode Active Material>

In the lithium secondary battery according to the present invention, asthe negative electrode active material, substances can be used whichhave a lithium dope-undope potential lower than that of theaforementioned lithium-iron composite phosphorus oxide. For example, dueto the reason that the potential of doing dope-undope lithium isappropriate, it is desirable to use a lithium-vanadium composite oxide,a transition metal chalcogenide, and the like.

Among them, due to the reason that stability in aqueous solutions isgood, it is desirable to use the lithium-vanadium composite oxide. Thelithium-vanadium composite oxide is such that the potential of doingdope-undope lithium ions reversibly is 2.2–3.0 V (vs. Li/Li⁺), and, incase of being combined with the aforementioned lithium-iron compositephosphorus oxide to use, it can secure a voltage close to 1 V.

In particular, even by repetitive charge-discharge, since itscrystalline structure is hardly broken down, and since it is possible tomaintain a large capacity, it is desirable to use the lithium-vanadiumcomposite oxide which has, in an X-ray diffraction pattern by means ofCuKα ray, the highest intensity peak at 2θ=13.9°±1° (θ being diffractionangle), and in which the intensity of the peak is 5 times or morecompared to the intensities of all of the other peaks.

The lithium-vanadium composite oxide is such that its composition is notlimited in particular, however, it can desirably be one which isexpressed by a composition formula Li_(x)V₃O_(y) (1.2<x<1.6;7.5≦y≦8.25). Those with this composition have merits in that theaforementioned specific crystalline structure can be readily obtained.

Describing the meaning of the composition range in detail, compared tothose with the aforementioned suitable range, in case of x≦1.2, theorientation in one direction in the crystalline structure lowers, and,moreover, in case of x≧1.6, impurities generate to bring about thelowering of battery capacity.

Commenting on the value of y, compared to those with the aforementionedsuitable range, in case of y<7.5, the capacity lowering, resulting fromthe defects in the crystalline structure, is likely to occur, and,moreover, in case of y>8.25, the probability of transformation intoanother crystalline structures enlarges.

The lithium-vanadium composite oxide having the aforementionedcomposition is such that its production method is not limited inparticular, however, in accordance with a later-described productionmethod, the lithium-vanadium composite oxide having the aforementionedcomposition can be produced with ease.

Moreover, as described above, from the viewpoint of showing asubstantially constant potential in charge-discharge and being able toreversibly dope-undope a large amount of lithium ions in a potentialrange where the hydrogen generation by means of the electrolysis ofwater does not arise, it is desirable to use a spinel structurelithium-vanadium composite oxide whose basic composition is LiV₂O₄.Commenting repeatedly, the basic composition means the representativecomposition of the lithium-vanadium composite oxide. Then, it is notnecessarily limited to the one with the stoichiometric composition, andthose with non-stoichiometric compositions and so forth, which ariseinevitably in the production, are included as well in which, forinstance, cations of the atoms, such as lithium, are omitted or theoxygen atoms are omitted.

This spinel structure lithium-vanadium composite oxide is such that itsproduction method is not limited in particular, however, in accordancewith a later-described production method discovered by the presentinventors, it can be produced with ease.

In addition, from the viewpoint that the potential of doing dope-undopelithium ions reversibly is 2–2.5 V (vs. Li/Li⁺) and, in case of beingcombined with the aforementioned positive-electrode-active-material rawmaterials to use, it can secure a voltage close to 2 V, and that it canconstitute a secondary battery of large capacity, it is desirable to usea transition metal chalcogenide as the negative electrode activematerial.

Above all, because of being less expensive and exhibiting largecapacities per unit weight of active materials, it is desirable to useTiS₂, MoS₂, NbS₂ and VS₂. One member of these can be used independently,and, moreover, two members or more can be mixed to use. In particular,due to the reason that it exhibits a large capacity, it is desirable touse TiS₂. Note that the aforementioned respective transition metalchalcogenides are such that their production methods are not limited inparticular, and they can be produced by the usually used methods.

<Positive Electrode and Negative Electrode>

Both of the positive electrode and negative electrode can be formed bymixing a conductor and a binder with the powdered respective activematerials, by pressing each of them, which are made into a positiveelectrode raw material mixture and a negative electrode raw materialmixture, onto a surface of an electricity collector, which is made ofmetal, or by coating and drying them thereon.

The conductor is for securing the electric conductivity of theelectrodes, and, for example, one member of carbonaceous-materialpowdered substances, such as carbon black, acetylene black and graphite,can be used, or two or more members thereof can be mixed to use.Moreover, the binder plays a role of fastening the active materialparticles and the conductor particles together, and it is possible touse, for instance, a fluorine-containing resin, such aspolytetrafluoroethylene, polyvinylidene fluoride and fluoroelastomer,and a thermoplastic resin, such as polypropylene and polyethylene.

<Aqueous Electrolytic Solution>

The electrolytic solution which is used in the lithium secondary batteryaccording to the present invention is an aqueous electrolytic solutionin which a lithium salt, acting as an electrolyte, is dissolved inwater. The lithium salt dissociates by dissolving in water, and turnsinto lithium ions to exist in the electrolytic solution. In general,oxide-based active-material raw materials exist more stably in aqueoussolutions of from neutrality to alkalinity. Moreover, in case of takingto furthermore activate the dope-undope reaction of lithium ion intoconsideration as well, it is desired that the using electrolyticsolution can be from neutral to alkaline. Note that being neutral hereinmeans that, in terms of the pH value, pH=6–8 approximately.

For example, in case of using a neutral electrolytic solution of pH=7,the hydrogen generation potential by means of the electrolysis of wateris 2.62 V, and the oxygen generation potential is 3.85 V (vs. Li/Li⁺),and in case of using an alkaline electrolytic solution of pH=14, thehydrogen generation potential is 2.21 V, and the oxygen generationpotential is 3.44 V (vs. Li/Li⁺). Namely, in case of using the neutralaqueous solution, since the oxygen generation potential by means of theelectrolysis of water is high, it is possible, as described above, forthe positive electrode active material to dope-undope a much more amountof lithium ions and to take out much larger capacity. Therefore, in caseof making a secondary battery of much larger capacity, it is desirableto use an electrolytic solution close to the neutrality, specifically,an electrolytic solution whose pH=6–10.

Moreover, in general, aqueous solutions are, compared to nonaqueoussolutions, better in terms of conductiveness, for example, a neutralaqueous solution has conductivity of 10 times or more as large as thatof a nonaqueous solution, and an alkaline aqueous solution hasconductivity of 100 times or more as large as that of a nonaqueoussolution. Accordingly, a secondary battery which uses an aqueoussolution as the electrolytic solution is, compared to a nonaqueoussecondary battery, of smaller internal resistance, especially, reactionresistance, and, in case of using an alkaline aqueous solution, theinternal resistance becomes much smaller. Therefore, in case of making asecondary battery of much better power characteristic and ratecharacteristic, it is desirable to use a strongly alkaline electrolyticsolution, specifically, an electrolytic solution whose pH=10–12.

The lithium salt which is usable as the electrolyte is, as far as itdissolves in water, not limited in particular, however, considering thestability and the like of the oxide being the positive electrode activematerial, it is desirable to use a lithium salt which turns theelectrolytic solution from neutrality to alkalinity after dissolving.Specifically, for example, it is desirable to use lithium nitrate,lithium hydroxide, lithium iodide and the like. These lithium salts canbe used independently, and, moreover, two members or more of these canbe used simultaneously. In particular, due to the reason that itexhibits high solubility and accordingly is of good conductiveness, inorder to make a neutral electrolytic solution, it is desirable to uselithium nitrate, and, moreover, in order to make a strongly alkalineelectrolytic solution, it is desirable to mix lithium nitrate andlithium hydroxide to use. Note that, due to the reason that it ispossible to enhance the electric conductivity of the electrolyticsolution to make the internal resistance of secondary batteries smaller,it is desired that the concentration of the lithium salt in theelectrolytic solution can be the saturation concentration, or aconcentration close to it.

<Other Constituent Elements etc.>

In the lithium secondary battery according to the present invention, anelectrode assembly is formed by facing the aforementioned positiveelectrode with the aforementioned negative electrode. It is desirable tointerpose a separator between the positive electrode and the negativeelectrode. This separator separates the positive electrode from thenegative electrode and holds the electrolytic solution, andcellulose-based ones can be used therefor.

The lithium secondary battery according to the present invention is suchthat its shape is not limited in particular, and it is possible to makeit into a variety of shapes, such as cylinder types, laminated types,coin types. In whatever case of employing any shape, it is possible tocomplete a lithium secondary battery by accommodating the aforementionedelectrode assembly, which is formed depending on the battery shape, in apredetermined battery case, by connecting the intervals from thepositive electrode electricity collector and negative electrodeelectricity collector to the positive electrode terminal and negativeelectrode terminal, which lead to the outside, with a lead and the likefor collecting electricity, by impregnating the aforementionedelectrolytic solution into this electrode assembly, and by enclosing itin the battery case.

<Production Methods of Respective Active Materials> (1) ProductionMethod of Hexagonal Layered Rocksalt Structure Lithium-ManganeseComposite Oxide

It has been general conventionally that the above-describedlithium-manganese composite oxide having a hexagonal layered rocksaltstructure is synthesized by ion-exchanging α-NaMnO₂, which issynthesized by a solid phase reaction method, in a nonaqueous solution,which includes lithium ions, at a temperature of 300° C. or less.However, since this method goes through the 2-stage complicated processsuch as the solid phase reaction method and ion-exchanging, it isdisadvantageous industrially.

Moreover, there is an example of synthesizing a lithium-manganesecomposite oxide having an orthorhombic layered rocksalt structure byhydrothermally reacting a manganese raw material with a water solublelithium salt in an aqueous solution which includes excessively ahydroxide of an alkaline metal other than lithium. However, the obtainedlithium-manganese composite oxide was, as charge-discharge was repeated,seen to transform into a spinel structure, and, in case of using it asthe active material of a nonaqueous secondary battery, it was notpossible to say that the cycle characteristic of the battery was good.

The present inventors repeated experiments, and discovered a method ofreadily producing the lithium-manganese composite oxide having ahexagonal layered rocksalt structure. The method is constituted byincluding: a dispersion aqueous solution preparation step of preparing adispersion aqueous solution by mixing manganese dioxide, making amanganese source, with a lithium hydroxide aqueous solution, making alithium source, so that Li/Mn is from 2 or more to 10 or less by molarratio; and a hydrothermal treatment step of heating and holding thedispersion aqueous solution at a temperature of from 120° C. or more to250° C. or less.

Namely, the present production method differs from the conventionalsolid phase reaction in that the hydrothermal treatment is carried out,and is a method in which the lithium-manganese composite oxide having ahexagonal layered rocksalt structure is obtained simply by dispersingthe manganese dioxide in the lithium hydroxide aqueous solution, and byheating and holding the dispersion aqueous solution. By using themanganese dioxide as a manganese source, it is possible to obtain thehexagonal layered rocksalt structure lithium-manganese composite oxidewhich hardly transforms into a spinel structure even aftercharge-discharge is repeated. Then, since it is possible for the presentproduction method to synthesize the aimed lithium-manganese compositeoxide by the 1-stage hydrothermal treatment, it makes a productionmethod which is easy and advantageous industrially. Hereinafter, therespective steps of the present production method will be described.

(a) Dispersion Aqueous Solution Preparation Step

The dispersion aqueous solution preparation step is a step in which adispersion aqueous solution is prepared by mixing manganese dioxide,making a manganese source, with a lithium hydroxide aqueous solutionwith such a ratio that Li/Mn is from 2 or more to 10 or less by molarratio.

The mixing ratio of the manganese dioxide with the lithium hydroxideaqueous solution is arranged to be such a ratio that Li/Mn is from 2 ormore to 10 or less by molar ratio. This is because, when Li/Mn is lessthan 2 by molar ratio, mixture phases with spinel structurelithium-manganese composite oxides are made, on the contrary, when itexceeds 10, the ratio of Li₂MnO₃, being an impurity, increases.

Moreover, mixing of the respective raw materials can be done so that themanganese dioxide is dispersed uniformly in the lithium hydroxideaqueous solution, and can be done by ordinary methods. For example, itis possible to name an ultrasonically dispersing method with anultrasonic homogenizer and the like, a dispersing method by giving ahigh shearing force with a homogenizer and so forth, and so on.

Note that, for the manganese dioxide used in a powdery form, it isdesirable to use one whose average particle diameter is from 0.1 μm ormore to 20 μm or less. This is because, when the average particlediameter is less than 0.1 μm, the particle diameters of the obtainedlithium-manganese composite oxides become so small that it is difficultto manufacture electrodes, when it exceeds 20 μm, the particle diametersof the lithium-manganese composite oxides become so large that it isdisadvantageous for power characteristic.

(b) Hydrothermal Treatment Step

The hydrothermal treatment step is a step in which the dispersionaqueous solution, prepared in the dispersion aqueous solutionpreparation step, is heated and held at a temperature of from 120° C. ormore to 250° C. or less. This is because, at temperatures of less than120° C., the reaction does not proceed, on the contrary, exceeding 250°C., the costs for withstanding pressure goes up.

The heating and holding time can be a time in which the reaction canterminate completely, and it can usually be carried out for 72 hoursapproximately. Moreover, it is possible to carry out the hydrothermaltreatment, for example, by putting the aqueous solution, obtained in thedispersion aqueous solution preparation step, in an autoclave withinside lined with Teflon, by holding it at the predetermined temperaturefor a predetermined time and by taking out products after cooling thetemperature to around room temperature by water-cooling or coolinggradually in the container.

The thus synthesized lithium-manganese composite oxide is taken out byfiltering it out of the aqueous solution, and is made into a powderedone by carrying out washing with water and drying. Note that the methodsof filtering, washing with water and drying are not limited inparticular, for example, specifically, it can be filtered out with asuction filtration apparatus and the like, can be washed with water bymeans of ultrasonic and so forth, and, moreover, after filteringsimilarly, can be dried with a drying furnace and so on at a temperatureof from 50 to 120° C. for a period of from 60 to 180 minutesapproximately.

The lithium-manganese composite oxide having a hexagonal layeredrocksalt structure, which is synthesized by the present productionmethod, is hardly transformed into a spinel structure even aftercharge-discharge is repeated, and a secondary battery, which uses thisas the active material, lowers the capacity less even aftercharge-discharge is repeated, namely, it makes a battery of very goodcycle characteristic.

Moreover, the lithium-manganese composite oxide having a hexagonallayered rocksalt structure, which is synthesized by the presentproduction method, is such that its application is not limited to thepositive electrode active material of aqueous lithium secondarybatteries, for example, it can be used as the active material and thelike of nonaqueous secondary batteries which use nonaqueous electrolyticsolutions.

At present, for the positive electrode active material of nonaqueouslithium secondary batteries, being readily synthesized and being handledwith relative readiness, in addition thereto, being good in terms ofcharge-discharge cycle characteristic, secondary batteries, which useLiCoO₂ as the positive electrode active material, are one of the mainstreams. However, since cobalt is less in view of the resource amount,the secondary batteries, which use LiCoO₂ as the positive electrodeactive material, are less likely to cope with the future mass-producingand upsizing that aim at electric sources for electric automobiles, andshould inevitably be expensive extremely in view of the cost. Hence, theaforementioned lithium-manganese composite oxide, which containsmanganese, being abundant as the resource and being less expensive, as aconstituent element, is expected to be a useful positive electrodeactive material which can substitute for cobalt. Therefore, the presentproduction method makes a method capable of readily producing thehexagonal layered rocksalt structure lithium-manganese composite oxidebeing a useful active material.

(2) Production Method of Olivine Structure Lithium-Iron CompositePhosphoric Acid Compound

It is possible to produce the above-described lithium-iron phosphoricacid compound, for example, by a method which comprises a raw-materialmixing step of obtaining a mixture by mixing raw materials, and acalcination step of calcining the mixture at a predeterminedtemperature. Hereinafter, the respective steps will be described.

(a) Raw-Material Mixing Step

The raw-material mixing step in the present production method of thelithium-iron composite phosphorus oxide is a step in which a mixture isobtained by mixing a lithium compound, an iron compound and aphosphorus-containing ammonium salt.

As for the lithium compound making a lithium source, it is possible touse Li₂CO₃, LiOH, LiOH.H₂O, LiNO₃, and the like. In particular, for thereason of exhibiting high reactivity, it is desirable to use LiOH.H₂O.

As for the iron compound making an iron source, as a compound in whichthe valence number of iron is divalent, it is possible to useFeC₂O₄.2H₂O, FeCl₂, and the like. In particular, for the reason ofgenerating gas, exhibiting low corrosiveness, in the calcination, it isdesirable to use FeC₂O₄.2H₂O.

As for the phosphorus-containing ammonium salt making a phosphorussource, it is possible to use NH₄H₂PO₄, (NH₄)₂HPO₄, P₂O₅, and the like.In particular, for the reason of exhibiting high reactivity, it isdesirable to use (NH₄)₂HPO₄.

All of the aforementioned raw materials can be used in a powdery form,and mixing them can be carried out by the methods which are used inmixing ordinary powders. Specifically, for example, they can be mixedwith a ball mill, a mixer, a mortar, and the like. Note that the mixingproportions of the respective raw materials can be arranged so as toconform to the compositions of the lithium-iron composite phosphorusoxides to be produced.

Moreover, in order to obtain the lithium-iron composite phosphorus oxidewhose average particle diameter is 1 μm or less, it is desirable tocontrol the average particle diameters of the aforementioned rawmaterials, in particular, it is desirable to use those whose averageparticle diameter is 1 μm or less for all of the respective rawmaterials.

(b) Calcination Step

The calcination step is a step in which the mixture, obtained in theraw-material mixing step, is calcined at a temperature of from 600° C.or more to 700° C. or less. In order to inhibit iron from being oxidizedto trivalent, the calcination can be carried out in an inert atmosphereor in a reducing atmosphere, specifically, for example, can be carriedout in an argon gas flow or a nitrogen gas flow, and the like.

The calcination temperature is arranged to be from 600° C. or more to700° C. or less. This because, when the calcination temperature is lessthan 600° C., the reaction does not proceed sufficiently so thatsub-phases, which are formed of other than the objective orthorhombiccrystalline one, are generated so that the crystallinity of thelithium-iron composite phosphorus oxide degrades. On the contrary, it isbecause, exceeding 700° C., the particles of the lithium-iron compositephosphorus oxide grow so that sufficient characteristic cannot beobtained. Moreover, in case of taking the uniformity of composition intoconsideration, the calcination can be, after preliminarily calcining atabout 350° C. for a predetermined time, carried out in theaforementioned temperature range. Note that the calcination time can bearranged to be a sufficient time for completing the calcination, and canbe usually carried out for 8 hours approximately.

The lithium-iron composite phosphorus oxide is, in case of using it asthe positive electrode active material of lithium secondary batteries,is generally used in a powdery form. Therefore, the one which isobtained by the calcination as aforementioned can be used for theproduction of batteries by carrying out pulverizing.

(3) Production Method of Lithium-Vanadium Composite Oxide Expressed byComposition Formula Li_(x)V₃O_(y) (1.2<x<1.6; 7.5≦y≦8.25)

It is possible to produce the lithium-vanadium composite oxide havingthe aforementioned composition, for example, by mixing a lithiumcompound, making a lithium source, with a vanadium compound, making avanadium source, with a predetermined proportion, and by calcining themixture in a predetermined atmosphere at a predetermined temperature.

As for the lithium compound making a raw material, it is possible to useLi₂CO₃, LiOH, LiNO₃, Li₂SO₄, and the like. As for the vanadium compound,it is possible to use V₂O₅, NH₄VO₃, and so forth. Note that, since V₂O₅has an advantage in that it makes a much less expensive raw material, itis further desirable to select V₂O₅ as the vanadium compound.

In this case, the mixing proportions of the aforementioned lithiumcompound and the aforementioned vanadium compound are arranged to besuch proportions that lithium and vanadium, included in each of them,conform to the composition ratios of the lithium-vanadium compositeoxides to be obtained.

The predetermined atmosphere in the calcination means slightly oxidizingatmospheres. For example, in case of using LiNO₃ or Li₂CO₃ as thelithium compound, since it generates oxidizing gas for itself incalcination, the calcination can be carried out in an argon gas flow.Thus, it is possible to create the aforementioned predeterminedatmosphere by calcining while preparing the types of flowing gas and theflow rate depending on the lithium compound and vanadium compound to bereacted.

The calcination temperature can, in case of using V₂O₅ as the vanadiumcompound, desirably be arranged to be from 600° C. to 750° C. In thistemperature range, since V₂O₅ enhances the reactivity by dissolving sothat a more homogenous lithium-vanadium composite oxide can be obtained,it is possible to calcine the lithium-vanadium composite oxide which issuitable for the negative electrode active material of the lithiumsecondary battery according to the present invention. Note that, due tothe reason that the growth of crystalline particles is inhibited so thatone having a large capacity can be obtained, the calcination temperaturecan further desirably be arranged to be from 600° C. to 680° C. Notethat the holding time at the aforementioned calcination temperature canbe arranged to be 3 hours or more approximately.

The lithium-vanadium composite oxide is, in case of using it as thenegative electrode active material of lithium secondary batteries, isgenerally used in a powdery form. Therefore, the one which is obtainedby the calcination as aforementioned can be used for the production ofbatteries by carrying out pulverizing.

(4) Production Method of Lithium-Vanadium Composite Oxide Having SpinelStructure

Lithium-vanadium composite oxides having a spinel structure have beenconventionally produced by mixing lithium compounds with vanadiumcompounds and by calcining their mixtures at high temperatures. However,since they have been calcined at high temperatures, it has beennecessary, in order to constitute secondary batteries having a practicalcapacity, to once pulverize the obtained lithium-vanadium compositeoxides and thereafter to use ones which are further heat-treated.Therefore, from the view point of reducing the number of productionsteps, production cost, and the like, it is desired that the calcinationis carried out at a relatively low temperature, however, in order tocarry out the calcination at a low temperature, it has been necessary tocarry it out in a strongly reducing atmosphere which contains hydrogen.

The present inventors repeated experiments, and discovered a method ofreadily producing the lithium-vanadium composite oxide having a spinelstructure. The method is constituted by including: a raw-material mixingstep of mixing a lithium compound, making a lithium source, a vanadiumcompound, making a vanadium source, and a carbon material, therebyobtaining a raw material mixture; and a calcination step of calciningsaid raw material mixture, thereby obtaining a lithium-vanadiumcomposite oxide.

Since the present production method is such that the carbon material ismixed in the raw material mixture, the atmosphere is, in thecalcination, turned into a reducing atmosphere by the carbon material.Namely, even in case of carrying out the calcination at a relatively lowtemperature, it is possible to readily produce the lithium-vanadiumcomposite oxide having a spinel structure without using hydrogen whichis difficult to handle. Moreover, in accordance with the presentproduction method, it is possible to produce the aforementionedlithium-vanadium composite oxide by a simple method in which therespective raw material compounds are mixed with the carbon materialwith predetermined proportions and the raw material mixture is calcined.Hereinafter, the respective steps will be described.

(a) Raw-Material Mixing Step

The present step is a step in which a lithium compound, making a lithiumsource, a vanadium compound, making a vanadium source, and a carbonmaterial are mixed, thereby obtaining a raw material mixture. As for thelithium compound, it is possible to use Li₂CO₃, LiOH, LiNO₃, LiSO₄, andthe like. As for the vanadium compound, it is possible to use V₂O₅,NH₄VO₃, and so forth. Note that, since V₂O₅ has an advantage in that itmakes a much less expensive raw material, it is further desirable toselect V₂O₅ as the vanadium compound. In this case, the mixingproportions of the aforementioned lithium compound and theaforementioned vanadium compound are arranged to be such proportionsthat lithium and vanadium, included in each of them, conform to thecompositions of the aiming lithium-vanadium composite oxides.

The carbon material is not limited in particular regarding its type. Forexample, it is possible to use by turning carbonaceous materials, likegraphitic materials, such as natural graphite, nodular or fibrousartificial graphite, easily-graphitized carbon, such as coke and carbonblack, difficultly-graphitized carbon, such as calcined phenolic resin,into a powdery form. Note that it is possible to independently use onemember of them, or to mix two members or more to use. Among them, incase of taking the dispersibility and the like in the raw materialmixture into consideration, it is desirable to use carbon black.

The mixing proportion of the carbon material can, in case of taking theentire weight of the mixture of the lithium compound and the vanadiumcompound as 100% by weight, desirably be arranged to be from 0.5% byweight or more to 5% by weight or less. This is because, in case ofbeing less than 0.5% by weight, compared to those within theaforementioned suitable range, the reduction operation is effected lessby the carbon material. On the contrary, exceeding 5% by weight,compared to those within the aforementioned suitable range, the mixingstate in the raw material mixture is less likely to be uniform, and,moreover, in case of constituting secondary batteries, it results inlowering the capacities. The mixed carbon material is such that its partis turned into carbon dioxide and the like to disappear by calcining theraw material mixture afterward. Note that the carbon material, remainingin the lithium-vanadium composite oxide, is considered to play a role ofa conductor.

All of the aforementioned respective raw materials can be used in apowdery form, and mixing them can be carried out by the methods whichare used in mixing ordinary powders. Specifically, for example, they canbe mixed with a ball mill, a mixer, a mortar, and the like.

(b) Calcination Step

The present step is a step in which the raw material mixture, obtainedin the aforementioned raw-material mixing step, is calcined, therebyobtaining a lithium-vanadium composite oxide. The calcinationtemperature can desirably be arranged to be from 600° C. or more to1,000° C. or less. This is because, when the calcination temperature isless than 600° C., the reaction does not fully proceed so that asingle-phase spinel phase cannot be obtained. On the contrary, exceeding1,000° C., it is turned in an excessively reducing state so thatvanadium compounds of low oxidation numbers have been generated. Inparticular, in case of taking suppressing the granular growth so as toinhibit the capacity from lowering into consideration, the calcinationtemperature can desirably be arranged to be 800° C. or less.

As described above, in case of calcining at a relatively low temperatureof 800° C. or less, it is necessary to calcine in a strongly reducingatmosphere which contains hydrogen, however, in the present productionmethod, the atmosphere is turned into a reducing atmosphere by thecarbon material in the raw material mixture during the calcination.Therefore, the calcination can be carried out in an inert gas flow, suchas an argon gas. Note that the calcination time can be arranged to be asufficient time for completing the calcination, and can usually becarried out for from 4 to 24 hours approximately.

<Permission of Other Embodiment Modes>

So far, the embodiment modes of the lithium secondary battery accordingto the present invention have been described, however, theabove-described embodiment modes are only a few of embodiment modes, andit is possible to carry out the lithium secondary battery according tothe present invention, beginning with the aforementioned embodimentmodes, in various modes which are subjected to modifications andimprovements based on the knowledge of a person having ordinary skill inthe art.

Further, the lithium secondary battery according to the presentinvention can be constituted by arbitrarily selecting the positiveelectrode active material and negative electrode active material,respectively, from the above-described materials. For example, it ispossible to employ a mode in which the hexagonal layered rocksaltstructure lithium-manganese composite oxide whose basic composition isLiMnO₂ is made into the positive electrode active material and thelithium-vanadium composite oxide, which has, in an X-ray diffractionpattern by means of CuKα ray, the highest intensity peak at 2θ=13.9°±1°(θ being diffraction angle), and in which the intensity of the peak is 5times or more compared to the intensities of all of the other peaks, ismade into the negative electrode active material. Furthermore, forinstance, it is possible to employ a mode in which the hexagonal layeredrocksalt structure lithium-manganese composite oxide whose basiccomposition is LiMnO₂ is made into the positive electrode activematerial and the spinel structure lithium-vanadium composite oxide whosebasic composition is LiV₂O₄ is made into the negative electrode activematerial. Moreover, it is possible to employ a mode in which the olivinestructure lithium-manganese composite oxide whose basic composition isLiFePO₄ is made into the positive electrode active material and thespinel structure lithium-vanadium composite oxide whose basiccomposition is LiV₂O₄ is made into the negative electrode activematerial.

EXPERIMENTAL EXAMPLES

Based on the aforementioned embodiment modes, a variety of lithiumsecondary batteries were manufactured in which the positive electrodeactive material, the negative electrode active material and the like arevaried, and the respective lithium secondary batteries were assessed.Hereinafter, the manufacture of the electrodes of the lithium secondarybatteries, and the assessment on the initial discharge capacities andcycle characteristics in the manufactured lithium secondary batterieswill be described.

<Manufacture of Electrodes of Lithium Secondary Batteries> (1)Manufacture of Positive Electrodes

As positive electrode active materials, a hexagonal layered rocksaltstructure lithium-manganese composite oxide expressed by a compositionformula LiMnO₂, a monoclinic zigzag layered structure lithium-manganesecomposite oxide expressed by a composition formula LiMnO₂, an olivinestructure lithium-iron composite phosphorus oxide expressed by acomposition formula LiFePO₄, and a layered rocksalt structurelithium-nickel composite oxide expressed by a compositional formulaLiNi_(0.8)Co_(0.15)Al_(0.05)O₂ were used respectively to manufacturepositive electrodes.

70 parts by weight of the aforementioned active-material raw materialswere mixed with 25 parts by weight of carbon as a conductor and 5 partsby weight of polytetrafluoroethylene as a binder, thereby obtainingpositive-electrode raw-material mixtures. Subsequently, 10 mg of thesepositive-electrode raw-material mixtures were bonded by pressing onto amesh, which had been welded onto inside a coin cell in advance and wasmade of stainless, with about 0.6 ton/cm²,thereby making positiveelectrodes. In following (a)–(d), the synthesis methods of therespective lithium composite oxides and lithium composite phosphorusoxide will be described.

(a) Synthesis of Hexagonal Layered Rocksalt Structure Lithium-ManganeseComposite Oxide

By the respective methods, the so-called hydrothermal method and solidphase method, two types of the lithium-manganese composite oxides weresynthesized.

(a-1) Synthesis by Hydrothermal Method

An LiOH aqueous solution was prepared by dissolving 6.29 g of LiOH.H₂Oin 80 ml of water. To this LiOH aqueous solution, MnO₂ was added in anamount of 2.61 g (Li/Mn becomes 5 by molar ratio), and was dispersedwith ultrasonic for 30 minutes, thereby preparing a dispersion aqueoussolution. Subsequently, this dispersion aqueous solution was put in anautoclave, and was reacted at a temperature of 200° C. for 7 days. Afterthe reaction, the autoclave was cooled, and the precipitates in thecontainer were filtered, washed with water and dried at 120° C., therebyobtaining a hexagonal layered rocksalt structure lithium-manganesecomposite oxide. Note that the obtained lithium-manganese compositeoxide was turned into a powdered one by pulverizing with a mortar.

(a-2) Synthesis by Solid Phase Method

Electrolytic manganese dioxide (MnO₂) and lithium carbonate (Li₂CO₃)were mixed by using a ball mill for 4 hours according to astoichiometric ratio. This mixture was heated to a calcinationtemperature of 900° C. at a temperature increment rate of 3.33° C./min.in an oxygen gas flow, was held at the temperature for 12 hours, and wasthereafter cooled gradually at a temperature decrement rate of 1°C./min. for a period of 15 hours, thereby obtaining a hexagonal layeredrocksalt structure lithium-manganese composite oxide. Note that theobtained lithium-manganese composite oxide was turned into a powderedone by pulverizing with a mortar.

(b) Synthesis of Monoclinic Zigzag Layered Structure Lithium-ManganeseComposite Oxide

An LiOH aqueous solution was prepared by dissolving 2.52 g of LiOH.H₂Oin 80 ml of water. To this LiOH aqueous solution, instead of the MnO₂,Mn₂O₃ was added in an amount of 2.37 g (Li/Mn becomes 2 by molar ratio),and was dispersed with ultrasonic for 30 minutes, thereby preparing adispersion aqueous solution. Subsequently, this dispersion aqueoussolution was put in an autoclave, and was reacted at a temperature of200° C. for 1 day. After the reaction, the autoclave was cooled, and theprecipitates in the container were filtered, washed with water and driedat 120° C., thereby obtaining a monoclinic zigzag layered structurelithium-manganese composite oxide. Note that the obtainedlithium-manganese composite oxide was turned into a powdered one bypulverizing with a mortar.

(c) Synthesis of Olivine Structure Lithium-Iron Composite PhosphorusOxide and Survey on Characteristic as Positive Electrode Active Material

LiOH.H₂O, FeC₂O₄.2H₂O and (NH₄)₂HPO₄ were mixed respectively so thatLi:Fe:P was 1:1:1 by molar ratio. The mixing was carried out with anautomatic mortar for 30 minutes. After preliminarily calcining thismixture at 350° C. for 5 hours, it was mixed with an automatic mortarfor 30 minutes. Thereafter, it was further calcined at 650° C. in anargon gas flow for 6 hours, thereby obtaining a lithium-iron compositephosphorus oxide. Note that the obtained lithium-iron compositephosphorus oxide was turned into a powdered one by pulverizing with amortar.

Here, an electrode was manufactured in which the present lithium-ironcomposite phosphorus oxide was made into the active material, and thecharacteristic as the positive electrode active material was surveyed bycarrying out a single electrode evaluation test by means of a cyclicvoltamogram. First, 70 parts by weight of the lithium-iron compositephosphorus oxide was mixed with 25 parts by weight of carbon as aconductor and 5 parts by weight of polytetrafluoroethylene as a binder,thereby obtaining an electrode raw-material mixture. Subsequently, 10 mgof this electrode raw-material mixture was bonded by pressing onto amesh, made of stainless, with a pressure of about 0.6 ton/cm², therebymaking an electrode.

Note that 2 types of the cyclic voltamogram were carried out by changingthe electrolytic solution. One of them was carried out by using anonaqueous electrolytic solution in which LiPF₆ was dissolved in aconcentration of 1M in a mixture solvent, in which ethylene carbonateand diethyl carbonate were mixed with a ratio of 3:7 by volume, and theother one of them was carried out by using an LiNO₃ aqueous solutionwith a saturated concentration in which LiNO₃, being a lithium salt, wasdissolved in water. Then, in case of the nonaqueous electrolyticsolution, both of the reference electrode and mating electrode werearranged to be metallic Li, in case of the aqueous electrolyticsolution, the reference electrode was arranged to be a silver-silverchloride electrode and the mating electrode was arranged to be aplatinum wire (φ0.3 mm×5 mm; coil shape), and a tri-polar beaker cellwas used respectively, and the scanning speed was arranged to be 2 mV/s.By this test, CV curves were obtained which showed the relationshipsbetween the currents and the potentials. The obtained CV curves areillustrated in FIG. 4. In FIG. 4, the solid line illustrates the resultsby using the aqueous electrolytic solution, and the dashed lineillustrates the results by using the nonaqueous electrolytic solution.

From FIG. 4, it was found out that the present lithium-iron compositephosphorus oxide shows a so-called 2-phase coexisting lithiumdope-undope behavior which has one and only oxidation-reductionpotential. Namely, different from LiNiO₂ and LiCoO₂ in which the latticeconstants and the like vary continuously with the dope-undope of lithiumto vary the potential in charge-discharge, the present lithium-ironcomposite phosphorus oxide shows a constant potential incharge-discharge. Then, the potential of charge-discharge is at around3.5 V (vs. Li/Li⁺), and this potential is within a potential range wherethe oxygen generation by means of the electrolysis of water does notarise.

Therefore, the present lithium-iron composite phosphorus oxide canreversibly dope-undope lithium ions in a large amount within a potentialrange where the oxygen generation by means of the electrolysis of waterdoes not arise, and it was possible to confirm that it is suitable asthe positive electrode active material in aqueous lithium secondarybatteries.

Moreover, in the CV curves illustrated in FIG. 4, the differenceresulting from the electrolytic solutions appears clearly in how thepeaks rise. In case of using the aqueous electrolytic solution, comparedto the case where the nonaqueous solution was used, it is understoodthat the rising of the peak is steeper and the polarization is less.Namely, it shows that, when the aqueous electrolytic solution is used,the reaction resistance in the oxidation-reduction reaction is less.

Exhibiting small reaction resistance means that, in case of constitutingbatteries by using the present lithium-iron composite phosphorus oxideas the positive electrode active material, the internal resistance ofthe batteries reduces in charge-discharge so that the powercharacteristics of the batteries are improved. Therefore, it wasconfirmed that the present lithium-iron composite phosphorus oxide ismore suitable for aqueous lithium secondary batteries.

(d) Synthesis of Layered Rocksalt Structure Lithium-Nickel CompositeOxide

By the respective methods, the so-called liquid phase method and solidphase method, two types of the lithium-nickel composite oxides weresynthesized.

(a-1) Synthesis by Liquid Phase Method

Respective 2M aqueous solutions of nickel nitrate, cobalt nitrate andaluminum nitrate were mixed so that Ni:Co:Al was 8:1.5:0.5 by molarratio, and were made into an aqueous solution of 500 mL. This aqueoussolution was dropped into a 4M sodium hydroxide aqueous solution, andhydroxide particles, containing nickel, cobalt and aluminum, wereprecipitated and synthesized. Then, the precipitated hydroxide particleswere filtered and washed, and were thereafter charged into water,thereby obtaining a hydroxide slurry. So as to arrange, with respect tothe sum (Ni+Co+Al) of Ni, Co and Al in the hydroxide slurry, the atomicratio of Li to be Li/(Ni+Co+Al)=1, a 3M lithium hydroxide aqueoussolution was added to the aforementioned slurry, thereby preparing adispersion liquid, and this dispersion liquid was dried by spraying in anitrogen gas atmosphere. After drying, the obtained composite oxideprecursors were calcined preliminarily at 350° C. in a nitrogenatmosphere for 1 hour, and were further calcined at 750° C. in an oxygenatmosphere for 12 hours, thereby obtaining a lithium-nickel compositeoxide.

(d-2) Synthesis by Solid Phase Reaction Method

LiOH.H₂O, Ni(OH)₂, Co₃O₄ and Al(OH)₃, making raw materials, were mixedrespectively so that Li, Ni, Co and Al were 1:0.8:0.15:0.05 by molarratio. Then, this mixture was calcined at a temperature of 900° C. in anoxygen gas flow for 24 hours, and, after cooling, it was pulverized,thereby obtaining a powdered lithium-nickel composite oxide.

(2) Manufacture of Negative Electrode

As negative electrode active materials, TiS₂, V₂O₅, LiMn₂O₄, alithium-vanadium composite oxide expressed by a composition formulaLi_(1.5)V₃O_(7.8-8), a spinel structure lithium-vanadium composite oxideexpressed by a composition formula LiV₂O₄, and a lithium-vanadiumcomposite oxide expressed by a composition formula LiV₃O₈ were usedrespectively to manufacture negative electrodes.

Similarly to the positive electrodes, 70 parts by weight of theaforementioned active-material raw materials were mixed with 25 parts byweight of carbon as a conductor and 5 parts by weight ofpolytetrafluoroethylene as a binder, thereby obtainingnegative-electrode raw-material mixtures. Subsequently, 10 mg of thesenegative-electrode raw-material mixtures were bonded by pressing onto amesh, which had been welded onto inside a coin cell in advance and wasmade of stainless, with about 0.6 ton/cm², thereby making negativeelectrodes. In following (a)–(c), the synthesis methods of therespective lithium-vanadium composite oxides and their characteristicsand the like as the negative electrode active materials will bedescribed.

(a) Synthesis of Lithium-Vanadium Composite Oxide Expressed byComposition Formula Li_(1.5)V₃O_(7.8-8) and Survey on Characteristic asNegative Electrode Active Material

1.688 g of lithium carobonate (Li₂CO₃) and 8.312 g of vanadium pentoxide(V₂O₅) were mixed with an automatic mortar for a period of 2 hours. Thismixture was heated to a calcination temperature of 680° C. at atemperature increment rate of 4.33° C./min. in an argon gas flow, washeld at the temperature for 12 hours, and was thereafter cooled to roomtemperature at a temperature decrement rate of 4.33° C./min., therebyobtaining a lithium-vanadium composite oxide. The obtainedlithium-vanadium composite oxide was, as a result of compositionanalysis, one whose composition was Li_(1.5)V₃O_(7.8-8). Moreover, thislithium-vanadium composite oxide was, in order to use it as the negativeelectrode active material, turned into a powdered one by pulverizingwith a mortar.

With respect to this lithium-vanadium composite oxide, an X-raydiffraction analysis by means of CuKα ray was carried out. An X-raydiffraction chart obtained as the result is illustrated in FIG. 2. Asdescribed above, in the X-ray diffraction chart illustrated in FIG. 2,it is possible to confirm that the highest peak is at 2θ≈13.9° (θ beingdiffraction angle) and the intensities of the other peaks, excepting thepeak, are extremely low, and, moreover, it is possible to confirm thatthe intensity of the highest peak is a value which is 5 times or more asmuch as the intensity values of all of the other peaks.

Moreover, an electrode was manufactured in which the presentlithium-vanadium composite oxide was made into the active material, andthe characteristic as the negative electrode active material wassurveyed by carrying out a single electrode evaluation test. First, 70parts by weight of the lithium-vanadium composite oxide were mixed with25 parts by weight of carbon as a conductor and 5 parts by weight ofpolytetrafluoroethylene as a binder, thereby obtaining an electroderaw-material mixture. Subsequently, 10 mg of this electrode raw-materialmixture was bonded by pressing onto a mesh, which was made of stainless,with a pressure of about 0.6 ton/cm², thereby making an electrode.

Next, by using a tri-polar beaker cell in which a silver-silver chlorideelectrode was made into the reference electrode and a platinum wire(φ0.3 mm×5 mm; coil shape) was made into the mating electrode, a singleelectrode evaluation test on the aforementioned electrode was carriedout. The single electrode evaluation test was arranged so thatcharge-discharge was carried out in a potential of from 0.265 V to –0.75V at a constant current whose current density was 2 mA/cm², and, by thistest, a charge-discharge curve (curve which shows the relationshipbetween capacities and potentials) in the range was obtained. Theobtained charge-discharge curve is illustrated in FIG. 5.

As it is apparent from FIG. 5, this charge-discharge potential range isa suitable potential range for the negative electrode active material ofaqueous lithium secondary batteries, and the charge-discharge curve isflat in the range, and the capacity per unit weight of active materialis also as large as 160 mAh/g. Therefore, it is possible to confirm thatthe present lithium-vanadium composite oxide is an active-material rawmaterial which exhibits good characteristic as the negative electrodeactive material of aqueous lithium secondary batteries.

Note that, as a result of carrying out a single electrode evaluationtest on TiS₂, used as the negative electrode active material, under theaforementioned conditions, although the theoretical capacity is said tobe 240 mAh/g (Li_(a)TiS₂: of 0≦a≦1), the capacity was 142 mAh/g in theactual measurement under these conditions. Moreover, showing thetheoretical capacities of V₂O₅ and LiMn₂O₄, being the other negativeelectrode active materials which have been investigated conventionally,they are 147 mAh/g (Li_(b)V₂O₅: of 0≦b≦1) for V₂O₅ and 148 mAh/g(Li_(c)Mn₂O₄: of 1≦c ≦2) for LiMn₂O₄, and it is possible to confirm thatthe present lithium-vanadium composite oxide is one which has a largercapacity than all of these negative-electrode-active-material rawmaterials. For reference, in Table 1 set forth below, these values aresummarized altogether.

TABLE 1 Capacity (mAh/g) Notes Li_(1.5)V₃O_(7.8–8) 160 Actually MeasuredValue TiS₂ 142 Actually Measured Value V₂O₅ 147 Theoretical ValueLiMn₂O₄ 148 Theoretical Value

(b) Synthesis of Lithium-Vanadium Composite Oxide Expressed byComposition Formula LiV₂O₄ and Survey on Characteristic as NegativeElectrode Active Material

First, lithium carbonate (Li₂CO₃) and vanadium pentoxide (V₂O₅) weremixed so that Li:V was 1:2 by molar ratio. The mixing was carried outwith an automatic mortar for 20 minutes. Subsequently, with respect to100 parts by weight of the mixture, Ketjen black was mixed in an amountof 2 parts by weight, thereby making a raw material mixture. The mixingwas carried out with an automatic mortar for 20 minutes. The rawmaterial mixture was calcined at 750° C. in an argon gas flow for 24hours, and was cooled rapidly, thereby obtaining a lithium-vanadiumcomposite oxide. Note that the obtained lithium-vanadium composite oxidewas, in order to use it as the negative electrode active material,turned into a powdered one by pulverizing with a mortar.

An electrode was manufactured in which the present lithium-vanadiumcomposite oxide was made into the active material, and thecharacteristic as the negative electrode material was surveyed bycarrying out a single electrode evaluation test. First, 70 parts byweight of the lithium-vanadium composite oxide were mixed with 25 partsby weight of carbon as a conductor and 5 parts by weight ofpolytetrafluoroethylene as a binder, thereby obtaining an electroderaw-material mixture. Subsequently, 10 mg of this electrode raw-materialmixture was bonded by pressing onto a mesh, which was made of stainless,with a pressure of about 0.6 ton/cm², thereby making an electrode.

Then, by using a tri-polar beaker cell in which a silver-silver chlorideelectrode was made into the reference electrode and, moreover, aplatinum wire (φ0.3 mm×5 mm; coil shape) was made into the matingelectrode, a single electrode evaluation test on the aforementionedelectrode was carried out in which a scanning speed was arranged to be 2mV/s. By this test, a CV curve is obtained which shows the relationshipbetween currents and potentials. The obtained CV curve is illustrated inFIG. 6.

From FIG. 6, it was found out that the present lithium-vanadiumcomposite oxide shows a so-called 2-phase coexisting lithium dope-undopebehavior which has one and only oxidation-reduction potential. Namely,different from LiNiO₂ and LiCoO₂ in which the lattice constants and thelike vary continuously with the dope-undope of lithium to vary thepotential in charge-discharge, the present lithium-vanadium compositeoxide shows a constant potential in charge-discharge. Then, thepotential of charge-discharge is at around 2.4 V (vs. Li/Li⁺), and thispotential is within a potential range where the hydrogen generation bymeans of the electrolysis of water does not arise. Therefore, thepresent lithium-vanadium composite oxide can reversibly dope-undopelithium ions in a large amount within a potential range where thehydrogen generation by means of the electrolysis of water does notarise, and it was possible to confirm that it is suitable as thenegative electrode active material in aqueous lithium secondarybatteries.

Moreover, a result of a single electrode evaluation test which wascarried out on the olivine structure lithium-iron composite phosphorusoxide, used as one of the positive electrode active materials andexpressed by a composition formula LiFePO₄, is also illustrated in FIG.7 together with the result of the present lithium-vanadium compositeoxide. From FIG. 7, the charge-discharge potential in the lithium-ironcomposite phosphorus oxide is at around 3.5 V (vs. Li/Li⁺), and thispotential is within a potential range where the oxygen generation bymeans of the electrolysis of water does not arise. Namely, it waspossible to confirm that an aqueous lithium secondary battery of about 1V-class can be constituted by using the lithium-iron compositephosphorus oxide, expressed by a composition formula LiFePO₄, as thepositive electrode active material and by using the presentlithium-vanadium composite oxide as the negative electrode activematerial.

(c) Synthesis of Lithium-Vanadium Composite Oxide Expressed byComposition Formula LiV₃O₈

Lithium carbonate (Li₂CO₃) and vanadium pentoxide (V₂O₅) were mixed sothat Li:V was 1:3 by molar ratio. The mixing was carried out with anautomatic mortar for 20 minutes. Subsequently, the mixture was heated byincreasing the temperature to 700° C. in an argon atmosphere, was heldat the temperature for 12 hours, and was thereafter cooled in thefurnace, thereby obtaining a lithium-vanadium composite oxide. Note thatthe obtained lithium-vanadium composite oxide was, in the same manner asaforementioned, turned into a powdered one by pulverizing with a mortar.

<Lithium Secondary Battery of First Series> (1) Manufacture of LithiumSecondary Battery (a) Lithium Secondary Battery of #11

A lithium secondary battery was manufactured in which the hexagonallayered rocksalt structure lithium-manganese composite oxide, obtainedby the hydrothermal method, was made into the positive electrode activematerial, and TiS₂ was made into the negative electrode active material.The aqueous lithium secondary battery was manufactured by facing, whileintervening a cellulose-based separator, the aforementioned positiveelectrode and negative electrode, including the respective activematerials, and by accommodating them, after impregnating them with anelectrolytic solution by injecting it in a predetermined amount, in thetype 2032 coin-shaped battery case (outside diameter 20 mm φ, thickness32 mm). The electrolytic aqueous solution was arranged to be a 5M LiNO₃aqueous solution which was made by dissolving LiNO₃, being a lithiumsalt, in water, and the pH was arranged to be about 7. The thusmanufactured aqueous lithium secondary battery was labeled as thelithium secondary battery of #11.

(b) Lithium Secondary Battery of #12

In the lithium secondary battery of #11, an aqueous solution, whose pHwas prepared to be 12 by adding LiOH to the 5M LiNO₃ aqueous solution,being an electrolytic solution, in a predetermined amount, was used asthe electrolytic solution. Excepting that, everything was arranged to bethe same as #11 to manufacture an aqueous secondary battery, and it waslabeled as a lithium secondary battery of #12.

(c) Lithium Secondary Battery of #13

In the lithium secondary battery of #11, except that the monocliniczigzag layered structure lithium-manganese composite oxide was made intothe positive electrode active material, everything was arranged to bethe same as #11 to manufacture an aqueous secondary battery. Themanufactured aqueous secondary battery was labeled as a lithiumsecondary battery of #13.

(d) Lithium Secondary Battery of #14

In the lithium secondary battery of #11, except that the electrolyticsolution was changed to a nonaqueous electrolytic aqueous solution;namely, one, in which LiPF₆ was dissolved in a mixture solvent, in whichethylene carbonate and diethyl carbonate were mixed by 3:7 by volumeratio, in a concentration of 1M, was used as the electrolytic solution,everything was arranged to be the same as #11 to manufacture an aqueoussecondary battery, and it was labeled as a lithium secondary battery of#14.

(e) Lithium Secondary Battery of #15

In the lithium secondary battery of #11, except that the layeredrocksalt structure lithium-nickel composite oxide, synthesized by thesolid phase method, was made into the positive electrode activematerial, everything was arranged to be the same as #11 to manufacturean aqueous secondary battery. The manufactured aqueous secondary batterywas labeled as a lithium secondary battery of #15.

(f) Lithium Secondary Battery of #16

In the lithium secondary battery of #11, except that the positiveelectrode active material was changed to the layered rocksalt structurelithium-nickel composite oxide, used in the aforementioned lithiumsecondary battery of #15, and further an aqueous solution, whose pH wasprepared to be 12 by adding LiOH to the 5M LiNO₃ aqueous solution, beingan electrolytic solution, in a predetermined amount, was used as theelectrolytic solution, everything was arranged to be the same as #11 tomanufacture an aqueous secondary battery, and it was labeled as alithium secondary battery of #16.

(g) Lithium Secondary Battery of #17

In the lithium secondary battery of #11, except that the negativeelectrode active material was changed; namely, V₂O₅was used, everythingwas arranged to be the same as #11 to manufacture an aqueous secondarybattery, and it was labeled as a lithium secondary battery of #17.

(2) Assessment on Initial Capacity

The aforementioned respective lithium secondary batteries of #11 through#17 were charged and discharged under the following two types ofconditions to measure the initial capacities. The first charge-dischargecondition was arranged so that, at 20° C., charging was carried out upto the charge upper limit voltage of 1.5 V at a constant current, whosecurrent density was 1.0 mA/cm², and subsequently discharging was carriedout down to the discharge lower limit voltage of 0.05 V at a constantcurrent, whose current density was 0.5 mA/cm². Moreover, the secondcharge-discharge condition was arranged so that, except that the chargeupper limit voltage was arranged to be 1.8 V, charge-discharge wascarried out in the same manner as the first condition. The initialcapacities of the respective lithium secondary batteries are set forthin Table 2.

TABLE 2 Positive Negative Initial Capacity (mAh/g) Electrode ElectrodeElectrolytic Charge Charge Active Active Solution TerminationTermination Material Material (pH Value) Voltage 1.5 V Voltage 1.8 V #11Layered Rocksalt Structure TiS₂ LiNO₃ (pH 7) 41.7 79.5 LiMnO₂ #12Layered Rocksalt Structure TiS₂ LiNO₃ + LiOH 30.1 34.6 LiMnO₂ (pH 12)#13 Zigzag Structure TiS₂ LiNO₃ (pH 7) 32.2 44.0 LiMnO₂ #14 LayeredRocksalt Structure TiS₂ LiPF₆/EC + DEC 11.0 21.7 LiMnO₂ #15 LayeredRocksalt Structure TiS₂ LiNO₃ (pH 7) Could Not Could NotLiNi_(0.8)C_(0.15)Al_(0.05)O₂ Be Charged Be Charged #16 Layered RocksaltStructure TiS₂ LiNO₃ + LiOH Could Not Could NotLiNi_(0.8)C_(0.15)Al_(0.05)O₂ (pH 12) Be Charged Be Charged #17 LayeredRocksalt Structure V₂O₅ LiNO₃ (pH 7) Could Not Could Not LiMnO₂ BeCharged Be Charged

In Table 2, those identified with “Could Not Be Charged” are the oneswhich did not reach the charge termination voltages after 9 hours passedfrom the start of charging. As it is apparent from Table 1, it waspossible for all of the secondary batteries of #11 through #13 to obtainlarge capacities. In particular, it is understood that the secondarybattery of #11, using the hexagonal layered rocksalt structurelithium-manganese composite oxide as the positive electrode activematerial, is of large capacity. This is because the layered rocksaltstructure lithium-manganese composite oxide, being the positiveelectrode active material, could, in a potential range where the oxygengeneration by means of the electrolysis of water did not arise,dope-undope lithium ions. Meanwhile, the lithium secondary batteries of#15 through #17 could not be charged. This is believed that the oxygenor hydrogen generation by means of the electrolysis of water arose sothat it was not possible to dope-undope lithium ions.

Note that, in FIG. 8, there are illustrated the variations of thepotentials of the lithium secondary batteries of #15 and #16 duringcharging. Note that, in FIG. 8, there is illustrated the variation ofthe potential of a nonaqueous secondary battery, using the same positiveelectrode and negative electrode as those of #15 and #16, and using anonaqueous solution in which LiPF₆ was dissolved in a mixture solvent,in which ethylene carbonate and diethyl carbonate were mixed by volumeratio of 3:7, in a concentration of 1M, with a solid line for reference.

From FIG. 8, the lithium secondary battery, using the nonaqueouselectrolytic solution, is such that the potential increases immediatelyafter starting charging, but the secondary batteries of #15 and #16 aresuch that the potential, after reaching a predetermined potential, doesnot increase more than that. In addition, the secondary battery of #16,using the electrolytic solution whose pH is 12, is of lower potentialthan the secondary battery of #15, using the electrolytic solution whosepH is 7. Namely, the higher the pH value is the lower the oxygengeneration potential is, and accordingly, in charging the secondarybatteries of #15 and #16, it is believed that it is highly probable thatoxygen generates at the positive electrode.

Moreover, the secondary battery of #11 is of considerably largercapacity than the secondary battery of #14. This is believed that, sinceaqueous solutions are better than nonaqueous solutions in terms ofconductivity, the secondary battery of #11, using the aqueous solutionas the electrolytic solution, was of lower internal resistance than thesecondary battery of #14, using the nonaqueous solution as theelectrolytic solution. Note that, in order to assess the internalresistances of the respective secondary batteries of #11 through #17,the impedance of each of the respective secondary batteries wasmeasured. The measurement method was such that the alternate-currentresistance at 1 kHz was measured by the 4-terminal method. The resultsof the measurement are set forth in Table 3.

TABLE 3 Negative Electrode Electrolytic Positive Electrode ActiveSolution Impedance Active Material Material (pH Value) (Ω) #11 LayeredRocksalt TiS₂ LiNO₃ (pH 7) 1.56 Structure LiMnO₂ #12 Layered RocksaltTiS₂ LiNO₃ + LiOH 1.46 Structure LiMnO₂ (pH 12) #13 Zigzag StructureTiS₂ LiNO₃ (pH 7) 1.93 LiMnO₂ #14 Layered Rocksalt TiS₂ LiPF₆/EC + DEC6.60 Structure LiMnO₂ #15 Layered Rocksalt TiS₂ LiNO₃ (pH 7) 1.21Structure LiNi_(0.8)Co_(0.15)Al_(0.05)O₂ #16 Layered Rocksalt TiS₂LiNO₃ + LiOH 1.08 Structure (pH 12) LiNi_(0.8)Co_(0.15)Al_(0.05)O₂ #17Layered Rocksalt V₂O₅ LiNO₃ (pH 7) 1.12 Structure LiMnO₂

As it is apparent from Table 3, the nonaqueous electrolytic solution,used in the secondary battery of #14, is of larger resistance than theaqueous electrolytic solutions, used in the other secondary batteries,and, compared to the secondary battery of #11, is about a quadrupleresistance value. Therefore, the lithium secondary batteries accordingto the present invention, using the aqueous solutions as theelectrolytic solution, are of large capacity, and, in addition, they areof small internal resistance and are good in terms of powercharacteristic and rate characteristic as well.

(3) Assessment on Cycle Characteristic

Next, on the lithium secondary battery of #11, a charge-discharge cycletest was carried out in order to assess the cycle characteristic. Thecharge-discharge cycle test was carried out at two types oftemperatures, 20° C., being room temperature, and 60° C., considered tobe the upper limit of actual service temperature range. The conditionswere arranged so that charge-discharge, in which charging was carriedout up to the charge termination voltage of 1.5 V at a constant current,whose current density was 1.0 mA/cm², and subsequently discharging wascarried out down to the discharge termination voltage of 0.05 V at aconstant current, whose current density was 0.5 mA/cm², was regarded asone cycle and this cycle was repeated by 100 cycles. Note that theintermission time between the charge and discharge was arranged to be 1minute. Moreover, in the same manner, a charge-discharge cycle test wascarried out in which the charge termination voltage was arranged to be1.8 V.

The discharge capacities at the respective cycles were measured, and theresults are illustrated in graphs of FIG. 9 and FIG. 10. FIG. 9 is agraph for illustrating the discharge capacities at 20° C. at therespective cycles, and, moreover, FIG. 10 is a graph for illustrating,in case of arranging the charge termination voltage to be 1.5 V, thedischarge capacities at 60° C. at the respective cycles. Note that, inFIG. 10, there is simultaneously illustrated the discharge capacities ofthe lithium secondary battery of #14, being a nonaqueous secondarybattery, discharge capacities which were measured by carrying out thecharge-discharge cycle test at 60° C. similarly.

As it is apparent from FIG. 9, it is understood that, regardless of thecharge termination voltages, the capacities decrease somewhat by theinitial 10 times of charge-discharge, but the subsequent capacitydecrements are small so that the cycle characteristics are good.Moreover, from FIG. 10, it is understood that the lithium secondarybattery of #11 is such that, even at a temperature as high as 60° C.,the capacity decrement by repeating charge-discharge is small, and thatits cycle characteristic is very good. Meanwhile, the lithium secondarybattery of #14 is such that the capacity decrement is considerable. Thisis believed that, since the lithium secondary battery of #14 uses theorganic solvent, being a nonaqueous solution, as the electrolyticsolution, its internal resistance is large and further it is susceptibleto the resistance increment accompanied by charge-discharge.

Therefore, it was possible to confirm that the lithium secondary batteryaccording to the present invention, in which the capacity decrement isless even after repeating charge-discharge, is of good cyclecharacteristic. Moreover, in particular, it was possible to confirm thatit is a secondary battery whose cycle characteristic is good at hightemperatures.

<Lithium Secondary Battery of Second Series> (1) Manufacture of LithiumSecondary Battery (a) Lithium Secondary Battery of #21

A lithium secondary battery was manufactured in which the hexagonallayered rocksalt structure lithium-manganese composite oxide, obtainedby the solid phase method, was made into the positive electrode activematerial, and the lithium-vanadium composite oxide, expressed by acomposition formula Li_(1.5)V₃O_(7.8-8), was made into the negativeelectrode active material. The aqueous lithium secondary battery wasmanufactured by facing, while intervening a cellulose-based separator,the aforementioned positive electrode and negative electrode, includingthe respective active materials, and by accommodating them, afterimpregnating them with an electrolytic solution by injecting it in apredetermined amount, in the type 2016 coin-shaped battery case (outsidediameter 20 mm φ, thickness 16 mm). The electrolytic solution was anLiNO₃ aqueous solution having a saturated concentration, which was madeby dissolving LiNO₃, being a lithium salt, in water, and the pH valuewas arranged to be about 7. The thus manufactured aqueous lithiumsecondary battery was labeled as the lithium secondary battery of #21.

(b) Lithium Secondary Batteries of #22 through #24

Lithium secondary batteries were manufactured in which only the negativeelectrode active materials were different from the aforementionedlithium secondary battery of #21 and the other constituent elements wereidentical therewith. The used negative electrode active materials werethose listed in the paragraph of the aforementioned single electrodeevaluation test on the lithium-vanadium composite oxides, expressed by acomposition formula Li_(1.5)V₃O_(7.8-8), the lithium secondary battery,using TiS₂, was labeled as the lithium secondary battery of #22, thelithium secondary battery, using V₂O₅, was labeled as the lithiumsecondary battery of #23, and the lithium secondary battery, usingLiMn₂O₄, was labeled as the lithium secondary battery of #24.

(2) Charge-Discharge Cycle Test

With respect to the aforementioned respective lithium secondarybatteries of #21 through #24, a charge-discharge cycle test was carriedout. The charge-discharge cycle test was arranged so that, at anenvironmental temperature of 20° C., charge-discharge, in which chargingwas carried out up the cell voltage of 1.2 V at a constant current,whose current density was 0.5 mA/cm², and thereafter discharging wascarried out down to the cell voltage of 0.1 V at a constant current,whose current density was 0.5 mA/cm², was regarded as one cycle and thiscycle was repeated by 100 cycles. Note that the charging intermissiontime and the discharging intermission time in the charge-discharge cyclewere arranged to be 1 minute, respectively. As the results of thischarge-discharge cycle test, the measured first time dischargecapacities per unit weight of the positive electrode active material ofthe respective lithium secondary batteries are set forth in Table 4below.

TABLE 4 First Time Discharge Negative Electrode Active Material Capacity(mAh/g) #21 Li_(1.5)V₃O_(7.8–8) 45.2 #22 TiS₂ 42.1 #23 V₂O₅ 25.5 #24LiMn₂O₄ 33.1

As it is apparent from aforementioned Table 4, it is understood that,compared to the lithium secondary battery of #23, using V₂O₅ as thenegative electrode active material, and to the lithium secondary batteryof #24, using LiMn₂O₄ as the negative electrode active material, thelithium secondary battery of #21, using Li_(1.5)V₃O_(7.8-8), and thelithium secondary battery of #22, using TiS₂, are such that theirdischarge capacities are large from the initial period ofcharge-discharge. Therefore, it is possible to confirm that the lithiumsecondary battery, using the lithium-vanadium composite oxide having theaforementioned specific crystalline structure for the negative electrodeactive material, is an aqueous lithium secondary battery of largecapacity.

The cycle characteristics of the lithium secondary batteries of #21 and#22 whose initial discharge capacities were large will be compared. Asanother result of the aforementioned charge-discharge cycle test, inFIG. 11, there are illustrated the discharge capacities per unit weightof the positive electrode active material at the respective cycles inthe charge-discharge cycle test on the two secondary batteries.

As it is apparent from FIG. 11, it is understood that, compared to thelithium secondary battery of #22 using TiS₂, the lithium secondarybattery of #21 using Li_(1.5)V₃O_(7.8-8) maintains much largercapacities even in case of repeating the cycle. It is believed that thecapacity decrement of the lithium secondary battery of #22 results fromthe fact that S in the active material is slightly unstable in aqueouselectrolytic solutions, and from the decomposition and the like of theelectrolytic solution by the potential decrement of the negativeelectrode. From this result, it was possible to confirm that the lithiumsecondary battery according to the present invention, which used thelithium-vanadium composite oxide having the aforementioned specificcrystalline structure for the negative electrode active material, is asecondary battery which is good in terms of cycle characteristic aswell.

<Lithium Secondary Battery of Third Series> (1) Manufacture of LithiumSecondary Battery (a) Lithium Secondary Battery of #31

A lithium secondary battery was manufactured in which the olivinestructure lithium-iron composite phosphorus oxide was made into thepositive electrode active material, and the lithium-vanadium compositeoxide, expressed by a composition formula Li_(1.5)V₃O_(7.8-8), was madeinto the negative electrode active material. The aqueous lithiumsecondary battery was manufactured by facing, while intervening acellulose-based separator, the aforementioned positive electrode andnegative electrode, including the respective active materials, and byaccommodating them, after impregnating them with an electrolyticsolution by injecting it in a predetermined amount, in the type 2016coin-shaped battery case. The electrolytic solution was an LiNO₃ aqueoussolution having a saturated concentration, which was made by dissolvingLiNO₃, being a lithium salt, in water, and the pH value was arranged tobe about 7. The thus manufactured aqueous lithium secondary battery waslabeled as the lithium secondary battery of #31.

(b) Lithium Secondary Batteries of #32 and #32

Two types of lithium secondary batteries were manufactured in which onlythe positive electrode active materials were different from theaforementioned lithium secondary battery of #31 and the otherconstituent elements were identical therewith. In one of them, thehexagonal layered rocksalt structure lithium-manganese composite oxide,obtained by the hydrothermal method, was made into the positiveelectrode active material, and the manufactured lithium secondarybattery was labeled as the lithium secondary battery of #32. In theother one of them, the layered rocksalt structure lithium-nickelcomposite oxide, obtained by the liquid phase method, was made into thepositive electrode active material, and the manufactured lithiumsecondary battery was labeled as the lithium secondary battery of #33.

(2) Charge-Discharge Cycle Test

With respect to the aforementioned respective lithium secondarybatteries of #31 through #33, a charge-discharge cycle test was carriedout. The charge-discharge cycle test was arranged so that, at anenvironmental temperature of 60° C., charge-discharge, in which chargingwas carried out up the cell voltage of 1.2 V at a constant current,whose current density was 0.5 mA/cm², and thereafter discharging wascarried out down to the cell voltage of 0.1 V at a constant current,whose current density was 0.5 mA/cm², was regarded as one cycle and thiscycle was repeated by 35 cycles. Note that the charging intermissiontime and the discharging intermission time in the charge-discharge cyclewere arranged to be 1 minute, respectively. As the results of thischarge-discharge cycle test, the measured first time dischargecapacities per unit weight of the positive electrode active materials ofthe respective lithium secondary batteries are set forth in Table 5below.

TABLE 5 First Time Discharge Positive Electrode Active Material Capacity(mAh/g) #31 LiFePO₄ 69.1 #32 LiMnO₂ 57.7 #33LiNi_(0.8)Co_(0.15)Al_(0.05)O₂ 18.5

As it is apparent from aforementioned Table 5, it is understood that,compared to the lithium secondary battery of #33, usingLiNi_(0.8)Co_(0.15)Al_(0.05)O₂ as the positive electrode activematerial, the lithium secondary battery of #31, using LiFePO₄ as thepositive electrode active material, and the lithium secondary battery of#32, using LiMnO₂ as the positive electrode active material, are suchthat their discharge capacities are large from the initial period ofcharge-discharge. This is because the lithium-iron composite phosphorusoxide and lithium-manganese composite oxide, being the positiveelectrode active materials, could dope-undope lithium ions in a largeamount in a potential range where the oxygen generation by means of theelectrolysis of water did not arise. Therefore, it was possible toconfirm that the lithium secondary batteries, using the lithium-ironcomposite phosphorus oxide and lithium-manganese oxide for the positiveelectrode active materials, are aqueous lithium secondary batteries oflarge capacities.

Moreover, as another result of the aforementioned charge-discharge cycletest, in FIG. 12, there are illustrated the discharge capacities perunit weight of the positive electrode active materials at the respectivecycles in the charge-discharge cycle test on the respective secondarybatteries. As it is apparent from FIG. 12, it is understood that, inparticular, the lithium secondary battery of #31, using LiFePO₄ as thepositive electrode active material, maintains larger capacities even incase of repeating the cycle. This is because the lithium-iron compositephosphorus oxide, being a positive electrode active material, is stablein aqueous solutions and it can reversibly dope-undope lithium ions.From this result, it was also possible to confirm that the lithiumsecondary battery according to the present invention, using thelithium-iron composite oxide, is a secondary battery which is good interms of cycle characteristic, in particular, cycle characteristic athigh temperatures.

<Lithium Secondary Battery of Fourth Series> (1) Manufacture of LithiumSecondary Battery (a) Lithium Secondary Battery of #41

A lithium secondary battery was manufactured in which the olivinestructure lithium-iron composite phosphorus oxide was made into thepositive electrode active material, and the lithium-vanadium compositeoxide, expressed by a composition formula LiV₂O₄, was made into thenegative electrode active material. The aqueous lithium secondarybattery was manufactured by facing, while intervening a cellulose-basedseparator, the aforementioned positive electrode and negative electrode,including the respective active materials, and by accommodating them,after impregnating them with an electrolytic solution by injecting it ina predetermined amount, in the type 2016 coin-shaped battery case. Theelectrolytic solution was an LiNO₃ aqueous solution having a saturatedconcentration, which was made by dissolving LiNO₃, being a lithium salt,in water, and the pH value was arranged to be about 7. The thusmanufactured aqueous lithium secondary battery was labeled as thelithium secondary battery of #41.

(b) Lithium Secondary Battery of #42

In the manufacture of the aforementioned lithium secondary battery of#41, except that the positive electrode active material was changed;namely, the hexagonal layered rocksalt structure lithium-manganesecomposite oxide, obtained by the hydrothermal method, was made into thepositive electrode active material, it was manufactured in the samemanner as the aforementioned lithium secondary battery of #41. Themanufactured lithium secondary battery was labeled as a lithiumsecondary battery of #42.

(c) Lithium Secondary Battery of #43

In the manufacture of the aforementioned lithium secondary battery of#41, except that the positive electrode active material was changed;namely, the layered rocksalt structure lithium-nickel composite oxide,obtained by the liquid phase method, was made into the positiveelectrode active material, it was manufactured in the same manner as theaforementioned lithium secondary battery of #41. The manufacturedlithium secondary battery was labeled as a lithium secondary battery of#43.

(d) Lithium Secondary Battery of #44

In the manufacture of the aforementioned lithium secondary battery of#41, except that the negative electrode active material was changed;namely, the lithium-vanadium composite oxide, expressed by acompositional formula LiV₃O₈, was made into the negative electrodeactive material, it was manufactured in the same manner as theaforementioned lithium secondary battery of #41. The manufacturedlithium secondary battery was labeled as a lithium secondary battery of#44.

(e) Lithium Secondary Battery of #45

In the manufacture of the aforementioned lithium secondary battery of#42, except that the negative electrode active material was changed;namely, the lithium-vanadium composite oxide, expressed by acompositional formula LiV₃O₈, was made into the negative electrodeactive material, it was manufactured in the same manner as theaforementioned lithium secondary battery of #42. The manufacturedlithium secondary battery was labeled as a lithium secondary battery of#45.

(2) Charge-Discharge Cycle Test

With respect to the aforementioned respective lithium secondarybatteries of #41 through #45, a charge-discharge cycle test was carriedout. The charge-discharge cycle test was arranged so that, at anenvironmental temperature of 60° C., charge-discharge, in which chargingwas carried out up the cell voltage of 1.4 V at a constant current,whose current density was 0.5 mA/cm², and thereafter discharging wascarried out down to the cell voltage of 0.1 V at a constant current,whose current density was 0.5 mA/cm², was regarded as one cycle and thiscycle was repeated by 50 cycles. Note that the charging intermissiontime and the discharging intermission time in the charge-discharge cyclewere arranged to be 1 minute, respectively.

As the results of the charge-discharge cycle test, the measured firsttime discharge capacities per unit weight of the positive electrodeactive materials of the respective lithium secondary batteries are setforth in Table 6 below. Note that, as a reference example, a secondarybattery was constituted by using LiMn₂O₄ as the negative electrodeactive material and the result of measuring its first time dischargecapacity is set forth therein simultaneously.

TABLE 6 Negative 1st Time Electrode Discharge Positive Electrode ActiveCapacity Active Material Material (mAh/g) #41 LiFePO₄ LiV₂O₄ 73.5 #42LiMnO₂ LiV₂O₄ 66.0 #43 LiNi_(0.8)Co_(0.15)Al_(0.05)O₂ LiV₂O₄ 26.5 #44LiFePO₄ LiV₃O₈ 63.2 #45 LiMnO₂ LiV₃O₈ 50.5 Reference LiFePO₄ LiMn₂O₄41.9 Example

As it is apparent from Table 6, it is understood that, compared to therespective lithium secondary batteries of #44 and 45, using LiV₃O₈ asthe negative electrode active material, the respective lithium secondarybatteries of #41 and #42, using LiV₂O₄ as the negative electrode activematerial, are such that their first time discharge capacities are large.Moreover, compared to the lithium secondary battery, using LiMn₂O₄ asthe negative electrode active material, their first time dischargecapacities are large considerably. Namely, it was possible to confirmthat the lithium secondary batteries according to the present invention,using the lithium-vanadium composite oxide expressed by a compositionformula LiV₂O₄ for the negative electrode active material, are aqueouslithium secondary batteries of large capacities. Note that the aqueouslithium secondary battery of #43 become one whose first time dischargecapacity was small. This is because the lithium-nickel composite oxidewas used for the positive electrode active material, and it is believedthat capacity could not be taken out too much within the aforementionedvoltage range. Therefore, as for the positive electrode active materialused in the lithium secondary battery according to the presentinvention, it is possible to say that the hexagonal layered rocksaltstructure lithium-manganese composite oxide, expressed by a compositionformula LiMnO₂, and the olivine structure lithium-iron compositephosphorus oxide, expressed by a composition formula LiFePO₄, aresuitable.

Moreover, as another result of the aforementioned charge-discharge cycletest, in FIG. 13, there are illustrated the discharge capacities perunit weight of the positive electrode active materials at the respectivecycles in the charge-discharge cycle test on the respective secondarybatteries of #41 through #45. As it is apparent from FIG. 13, it isunderstood that the respective lithium secondary batteries of #41 and#42, using LiV₂O₄ as the negative electrode active material, maintain,compared to the respective lithium secondary batteries of #44 and #45,using LiV₃O₈ as the negative electrode active material, largercapacities even in case of repeating the cycle. From this result, it waspossible to confirm that the lithium secondary batteries according tothe present invention, using the lithium-vanadium composite oxideexpressed by a composition formula LiV₂O₄ for the negative electrodeactive material, are good aqueous lithium secondary batteries in termsof cycle characteristic.

INDUSTRIAL APPLICABILITY

The lithium secondary battery according to the present invention is anaqueous lithium secondary battery which uses an aqueous solution for theelectrolytic solution, and makes a secondary battery, which is lessexpensive, whose safety is extremely high and which is of high power aswell as large capacity, by using a suitable positive-electrodeactive-material raw material. Moreover, even after repeatingcharge-discharge, it makes a secondary battery whose capacity decrementis small, and which is good in terms of cycle characteristic, inparticular, cycle characteristic at high temperatures.

Such a lithium secondary battery according to the present invention canbe used widely, in addition to the fields of communication appliancesand information-related appliances, as an electric source and the likefor electric automobiles whose developments have been urged because ofthe environmental problems as well as the resource problems. Inparticular, it is useful as an electric source for powering automobilesand so forth which are expected to be used under severe conditions suchas the service temperatures.

1. A lithium secondary battery comprising: a positive electrode formedby binding a positive-electrode raw-material mixture including apositive electrode active material, a negative electrode formed bybinding a negative-electrode raw-material mixture including a negativeelectrode active material, and an electrolytic solution comprising anaqueous solution in which a lithium salt is dissolved, wherein saidpositive electrode active material is an olivine structure lithium-ironcomposite phosphorus oxide whose basic composition is LiFePO₄, andwherein said battery is adapted for impregnating said positive electrodeand said negative electrode with said electrolytic solution.
 2. Thelithium secondary battery set forth in claim 1, wherein said negativeelectrode active material comprises a lithium-vanadium composite oxide.3. The lithium secondary battery set forth in claim 1, wherein saidnegative electrode active material comprises a transition metalchalcogenide.
 4. A lithium secondary battery comprising: a positiveelectrode formed by binding a positive-electrode raw-material mixtureincluding a positive electrode active material, a negative electrodeformed by binding a negative-electrode raw-material mixture including anegative electrode active material, and an electrolytic solutioncomprising an aqueous solution in which a lithium salt is dissolved,wherein said positive electrode active material comprises (1) a layeredstructure lithium-manganese composite oxide whose basic composition isLiMnO₂, (2) an olivine structure lithium-iron composite phosphorus oxidewhose basic composition is LiFePO₄, or a combination of (1) and (2),wherein said negative electrode active material comprises alithium-vanadium composite oxide, and wherein said battery is adaptedfor impregnating said positive electrode and said negative electrodewith said electrolytic solution.
 5. The lithium secondary battery setforth in claim 4, wherein said lithium-vanadium composite oxide has, inan X-ray diffraction pattern by means of CuKα ray, the highest intensitypeak at 2θ=13.9°±1° (θ being diffraction angle), and the intensity ofthe peak is 5 times or more compared to the intensities of all of theother peaks.
 6. The lithium secondary battery set forth in claim 5,wherein said lithium-vanadium composite oxide is expressed by acomposition formula Li_(x)V₃O_(y)(1.2<x<1.6; 7.5≦y≦8.25).
 7. The lithiumsecondary battery set forth in claim 4, wherein said lithium-vanadiumcomposite oxide is a spinel structure lithium-vanadium composite oxidewhose basic composition is LiV₂O₄.
 8. The lithium secondary battery setforth in claim 4, wherein said lithium-manganese composite oxideincludes a lithium-manganese composite oxide (space group C2/m) havingan orthorhombic layered structure or a lithium-manganese composite oxide(space group Pmnm) having a monoclinic zigzag layered structure.
 9. Alithium secondary battery comprising: a positive electrode formed bybinding a positive-electrode raw-material mixture including a positiveelectrode active material. a negative electrode formed by binding anegative-electrode raw-material mixture including a negative electrodeactive material, and an electrolytic solution comprising an aqueoussolution in which a lithium salt is dissolved, wherein said positiveelectrode active material comprises (1) a layered structurelithium-manganese composite oxide whose basic composition is LiMnO₂, (2)an olivine structure lithium-iron composite phosphorus oxide whose basiccomposition is LiFePO₄, or a combination of (1) and (2), wherein saidnegative electrode active material comprises a transition metalchalcogenide, and wherein said battery is adapted for impregnating saidpositive electrode and said negative electrode with said electrolyticsolution.
 10. The lithium secondary battery set forth in claim 9,wherein said transition metal chalcogenide is at least one selected fromthe group consisting of TiS₂, MOS₂, NbS₂ and VS₂.
 11. The lithiumsecondary battery set forth in claim 9 wherein said lithium-manganesecomposite oxide includes a lithium-manganese composite oxide (spacegroup C2/m) having an orthorhombic layered structure or alithium-manganese composite oxide (space group Pmnm) having a monocliniczigzag layered structure.